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United States Patent |
5,538,617
|
Steinbicker
,   et al.
|
July 23, 1996
|
Ferrocyanide-free halogen tin plating process and bath
Abstract
Incorporating an additive into a tin electroplating bath substantially
inhibits soluble ferrous ions, ferric ions, and stannous ions from
reacting thus minimizing the formation of stannic tin which is lost in the
plating sludge.
Inventors:
|
Steinbicker; Richard N. (Allentown, PA);
Yau; Yung-Herng (Allentown, PA);
Fodor; Edward S. (Northampton, PA)
|
Assignee:
|
Bethlehem Steel Corporation ()
|
Appl. No.:
|
400941 |
Filed:
|
March 8, 1995 |
Current U.S. Class: |
205/302; 205/101; 205/253 |
Intern'l Class: |
C25D 003/22 |
Field of Search: |
205/302,253,101
|
References Cited
U.S. Patent Documents
2402185 | Jun., 1946 | Schweikher | 204/54.
|
2407579 | Sep., 1946 | Schweikher | 204/54.
|
2457152 | Dec., 1948 | Hoffman | 204/54.
|
2461507 | Feb., 1949 | Gray et al. | 204/54.
|
2512719 | Jun., 1950 | Hull | 204/54.
|
2585902 | Feb., 1952 | Gray | 204/54.
|
2736692 | Feb., 1956 | Eckert | 204/54.
|
2758075 | Aug., 1956 | Swalheim | 204/28.
|
3031400 | May., 1960 | Tsu | 205/302.
|
3518171 | Jun., 1970 | Merker et al.
| |
3849263 | Nov., 1974 | Gedde | 205/302.
|
3887444 | Jun., 1975 | Fueki et al. | 204/43.
|
3907653 | Sep., 1975 | Horn | 204/94.
|
3915812 | Oct., 1975 | Yamagishi et al. | 204/34.
|
3926759 | Dec., 1975 | Horn et al. | 204/151.
|
4006213 | Feb., 1977 | Fisher et al. | 204/54.
|
4073701 | Feb., 1978 | Steinbicker et al. | 204/54.
|
4163700 | Aug., 1979 | Igarashi et al. | 204/54.
|
4329207 | May., 1982 | Maruta | 204/54.
|
4459185 | Jul., 1984 | Obata et al. | 204/53.
|
4702383 | Oct., 1987 | Babcock et al. | 210/638.
|
4717460 | Jan., 1988 | Nobel et al. | 204/444.
|
5006367 | Apr., 1991 | Lancsek | 427/129.
|
5094726 | Mar., 1992 | Nobel et al. | 205/254.
|
5108615 | Apr., 1992 | Hosea et al. | 210/668.
|
5110423 | May., 1992 | Little et al. | 205/254.
|
5118394 | Jun., 1992 | Makino et al. | 205/253.
|
5174887 | Dec., 1992 | Federman et al. | 205/302.
|
5178746 | Jan., 1993 | Darnall et al. | 205/287.
|
5185076 | Feb., 1993 | Yanada et al. | 205/252.
|
5296128 | Mar., 1994 | Gernon et al. | 205/302.
|
5298168 | Mar., 1994 | Guess | 210/713.
|
5378347 | Jan., 1995 | Thomson et al. | 205/302.
|
Foreign Patent Documents |
60389/86 | Feb., 1987 | AU.
| |
Primary Examiner: Niebling; John
Assistant Examiner: Mayekar; Kishor
Claims
What is claimed:
1. A weakly halogen acidic plating bath free of ferrocyanide ions for
high-speed tin electroplating of steel comprising a conductive
electrolyte, soluble ferrous ions, ferric ions, and stannous ions together
with an effective amount of an additive selected from the group consisting
of para-aminobenzoic acid, gallic acid, catechol, resorcinol, salicylic
acid, citric acid, oxalic acid, formic acid, acetic acid, tartaric acid
glycine, diethyl hydroxylamine (DEHA), mixtures of citric acid and
hydroquinone, and mixtures thereof to substantially inhibit soluble
ferrous ions, ferric ions, and stannous ions from reacting, to minimize
the formation of stannic tin.
2. A plating bath as recited in claim 1 wherein said plating bath has a pH
in the range of greater than 0 to about 6.
3. A plating bath as recited in claim 2 wherein said plating bath has a pH
in the range of from about 3 to about 4.
4. A method for minimizing the effect of dissolved iron in high-speed tin
electroplating using a weakly acidic plating bath solution free of
ferrocyanide comprising:
adding to a halogen plating bath containing a conductive electrolyte,
soluble ferrous ions, ferric ions, and stannous ions, an additive selected
from the group consisting of para-aminobenzoic acid, gallic acid,
catechol, resorcinol, salicylic acid, citric acid, oxalic acid, formic
acid, acetic acid, tartaric acid, glycine, diethyl hydroxylamine (DEHA),
mixtures of citric acid and hydroquinone, and mixtures thereof in an
amount sufficient to substantially inhibit soluble ferrous ions, ferric
ions, and stannous ions from reacting, to minimize the formation of
stannic tin.
5. A method for minimizing the effect of dissolved iron in high-speed tin
electroplating as recited in claim 4 further comprising the step of
removing said ferrous ions from said halogen plating bath by recirculating
a portion of said bath through an iron removal apparatus.
6. A method for minimizing the effect of dissolved iron in high-speed tin
electroplating as recited in claim 4 wherein said halogen plating bath
solution has a pH in the range of greater than 0 to 6.
7. A method for minimizing the effect of dissolved iron in high-speed tin
electroplating as recited in claim 4 wherein said halogen plating bath
solution has a pH in the range of from about 3 to about 4.
8. A weakly acidic halogen plating bath solution free of ferrocyanide for
use as a tin electroplating bath, said bath containing soluble ferrous
ions, ferric ions, and stannous ions and an additive to inhibit said ions
from reacting to form stannic tin, said additive selected from the group
consisting of para-aminobenzoic acid, gallic acid, catechol, resorcinol,
salicylic acid, citric acid, oxalic acid, formic acid, acetic acid,
tartaric acid, glycine, diethyl hydroxylamine (DEHA), mixtures of citric
acid and hydroquinone, and mixtures thereof.
Description
FIELD OF THE INVENTION
The present invention relates to a process for the high-speed tin plating
of steel and a plating bath composition which is free of ferrocyanides.
BACKGROUND OF THE INVENTION
One of several known processes for the production of tinplated steel,
high-speed halogen electroplating, typically uses plating baths which
comprise stannous chloride, sodium bifluoride, sodium fluoride, sodium
chloride and hydrochloric acid together with a grain-refining additive. In
order to minimize corrosion of the steel as it is plated, however, the
degree of acidity must be moderate (i.e., pH of between 3 to 4). Moderate
acidity, in turn, requires that the stannous tin be combined with fluoride
ions in a chemical complex in order to minimize the reaction of stannous
tin with oxygen to form stannic tin which precipitates and is lost in the
plating sludge. Dissolved iron in the plating bath accelerates the
oxidation of fluoride-complexed stannous ions so that a substantial
portion of the stannous ions are lost in the plating sludge. Thus, the
iron, if not removed from the halogen plating bath, de-stabilizes the
process resulting in off-quality tinplate, low productivity, and high
costs for replenishment of tin and other chemicals.
To counteract the effect of iron, ferrocyanide is added in large quantities
to the bath to remove dissolved iron from the electrolyte by forming
insoluble compounds which report to the plating sludge. It is normally
added as the sodium ferrocyanide decahydrate salt and results in the
immediate and total precipitation of iron ions from the electrolyte before
they react with oxygen and/or stannous tin. The use of alkali
ferricyanides or ferrocyanides for this purpose is disclosed in U.S. Pat.
Nos. 2,402,185 and 2,512,719.
These ferrocyanide additions result, however, in precipitated iron
ferrocyanides which, along with insoluble stannic tin in the form of
sodium fluostannate, become the major ingredients of a heavy sludge that
accumulates in the plating cells, storage tanks, and throughout the
recirculating system. Typically, halogen electroplating lines must be shut
down periodically so that this sludge can be removed. In recent years,
concern has been growing about the environmental impact of the
ferrocyanide content of the plating section sludge, slurries, and waste
waters.
Thus, there is a need to develop other ways to remove iron and/or stabilize
a halogen plating bath so that the use of ferrocyanide can be
discontinued.
SUMMARY OF THE INVENTION
The present invention relates to a weakly acidic halogen plating bath
solution free of ferrocyanides for the electroplating of tin on an
iron-based substrate. The plating bath solution contains a conductive
electrolyte, stannous ions, and an effective amount of an additive
incorporated into the plating bath sufficient to substantially inhibit
soluble ferrous ions, ferric ions, and stannous ions from reacting in
solution. This solution minimizes the formation of stannic tin, Sn(IV).
The additive is preferably selected from the group consisting of
para-aminobenzoic acid, hydroquinone, gallic acid, catechol, resorcinol,
salicylic acid, ascorbic acid (L- or D-), citric acid, oxalic acid, formic
acid, acetic acid, tartaric acid, glycine, diethyl hydroxylamine (DEHA), a
mixture of citric acid and hydroquinone, and mixtures thereof.
Also disclosed is a method for minimizing the effect of dissolved iron in
high-speed tin electroplating without the need for ferrocyanides which
uses a weakly acidic halogen bath solution containing an additive in an
amount sufficient to substantially inhibit soluble ferrous ions, ferric
ions, and stannous ions from reacting in solution, to minimize the
formation of stannic tin.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a schematic arrangement of a portion of a high-speed halogen
plating process;
FIG. 2 is a graph showing the effect of ferrous iron on the oxidation of
stannous ions in a conventional halogen bath without ferrocyanides or
other additives;
FIG. 3 is a graph showing the effect of certain carboxylic acid addition
agents on the oxidation of stannous ions in a halogen bath in accordance
with the present invention;
FIG. 4 is a graph showing the effect of certain aromatic compound addition
agents on the oxidation of stannous ions in a halogen bath in accordance
with the present invention;
FIG. 5 is a graph showing the effect of L-ascorbic acid and D-ascorbic acid
addition agents on the oxidation of stannous ions in a halogen bath in
accordance with the present invention;
FIG. 6 is a graph showing the effect of certain aliphatic compound addition
agents on the oxidation of stannous ions in a halogen bath in accordance
with the present invention; and
FIG. 7 is a graph showing the confined effect of hydroquinone and citric
acid addition agents on the oxidation of stannous ions in a halogen bath
in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention uses chemical additives to stabilize halogen
electroplating baths so that they may be used without ferrocyanide
additions in the high-speed halogen tinplating of steel. FIG. 1 shows a
horizontal, high-speed, halogen line having a cleaning tank 10, a first
water rinsing unit 20, a surface activation unit 30, a second water
rinsing unit 40, and a plating section 50.
A ferrous substrate, e.g. coiled steel 52, to be plated is first pretreated
by cleaning and surface activation operations. These are accomplished by
sequentially passing the substrate 52 through cleaning tank 10, first
water rinsing unit 20, surface activation unit 30, and second water
rinsing unit 40. The substrate 52 is then passed through plating section
50 which comprises a plurality of horizontal plating cells arranged on two
plating decks, one above the other. The bottom side 54 of the substrate 52
is plated in the floor-level cells 56, and then the substrate 52 is
diverted to the second level 58 where it reverses direction and passes
through additional cells for plating the top side 60, which, because of
the reversal, is actually the bottom surface in those plating cells. The
horizontal plating cell geometry and high strip speeds result in a high
degree of aeration of the plating bath. After exiting the plating section
50, the steel is given further processing according to well-known
technology which is not shown, to result in a coil of tin plated steel
that is ready for shipment to a customer.
The geometry of the halogen process equipment dictates the chemical
requirements of the plating bath to be used. When using horizontal plating
cells the degree of acidity must be moderate (i.e., a pH of from greater
than zero up to about 6 and preferably between about 3 to about 4) in
order to minimize corrosion, especially of the top surface of the steel
strip as it passes through the first tier plating cells. In order to
perform a high-speed tinplating operation, an acid plating bath, based on
stannous rather than stannic ions, is required. The moderate acidity of
the bath requires that the stannous ions (Sn.sup.+2) in the bath be
combined with fluoride ions in an anionic complex (SnF.sub.3.sup.-1). In
this form, the reaction between dissolved oxygen resulting from a high
degree of aeration of the bath and stannous tin is relatively slow, but it
does result in the formation of tetravalent or stannic tin ion
(Sn.sup.+4). The stannic ion also combines with fluoride ions to form a
complex fluostannate anion (SnF.sub.6.sup.-2) which has limited solubility
and precipitates from the bath as a sodium salt (Na.sub.2 SnF.sub.6).
It is believed that the effect of iron on the depletion of the stannous
ions in the halogen bath occurs according to the following series of
chemical reactions:
4Fe.sup.+2 +4H.sup.+ +O.sub.2 .fwdarw.4Fe.sup.+3 +2H.sub.2 O(1)
Fe.sup.+3 +3Na.sup.+ +6F.sup.- .fwdarw.Na.sub.3 FeF.sub.6 (s).dwnarw.(2)
2Fe.sup.+3 +Sn.sup.+2 .fwdarw.2Fe.sup.+2 +Sn.sup.+4 (3)
2Sn.sup.+2 +O.sub.2 +4H.sup.+ .fwdarw.2Sn.sup.+4 +2H.sub.2 O(4)
Sn.sup.+4 +2Na.sup.+ +6F.sup.- .fwdarw.Na.sub.2 SnF.sub.6 (s).dwnarw.(5)
Reaction 1 illustrates how free ferrous ions (Fe.sup.+2) which enter an
aerated plating bath react rapidly with the dissolved oxygen to form
ferric ions (Fe.sup.+3). These ferric ions then follow one of two courses:
they either react with sodium and fluoride to form insoluble
sodiumfluoferrate according to reaction 2 or they react with stannous tin
to produce stannic tin according to reaction 3 (during which ferric ions
are reduced back to the ferrous ions). Stannic tin will also be produced
by the direct reaction of stannous tin with oxygen according to reaction
4.
In a system where no additional iron is introduced, the effect of reaction
3 (which occurs very quickly) would quickly dissipate with elimination of
ferric ions by tile formation of sodium fluoferrate (Na.sub.3 FeF.sub.6)
which precipitates to the sludge according to reaction 2.
In a continuous plating line, however, iron contamination of the plating
bath is persistent. It is believed that the majority of iron enters the
conventional plating bath in three different ways in a typical halogen
tinplating process. The first is by drag-in from the surface-activation
step (acid treatment and water rinse) which precedes the plating process.
The second is by dissociation of ferrocyanide ions to ferrous ions and
cyanide ions. Corrosion of the steel strip top surface as the bottom is
being plated is the third way.
Corrosion is believed to be the smallest contributor, mainly because iron
is more noble than tin in the complex chemistry of the halogen bath. As
soon as any tin is plated on the bottom surface, and especially as the
deposit wraps around onto the top at the strip edges, the steel is
galvanically protected and this greatly retards the dissolution of iron
from the unplated top surface.
On the other hand, the effect of the dissociation of ferrocyanide ions is
believed to be significant, depending on pH, temperature, and ferrocyanide
concentration. In a ferrocyanide-free electrolyte, however, such
dissociation would not be a source of iron. Thus, by discontinuing
ferrocyanide additions, the most significant source of iron will be
drag-in from the pre-treatment section. Although improvements can be made
to reduce the amount of iron dragged into the bath, the problem cannot be
totally avoided. As a result, reactions 1 and 3 take place continuously at
rapid rates to produce the stannic tin ion.
Stannic tin ions are also produced at lower rates according to reaction 4.
The stannic ions ultimately precipitate to the sludge as sodium
fluostannate according to reaction 5. This continuous precipitation leads
to a rapid decrease in the stannous concentration and a consequent
decrease in the limiting current density of the plating process. Without
the use of ferrocyanide to immediately precipitate iron, the stannous
concentration may drop to a level where it is difficult or impossible to
produce a high-quality tin coating. Moreover, to compensate for the
precipitation of sodium fluostannate, higher tin and chemical
replenishments must be made to the plating bath, thus resulting in higher
operating costs.
Thus, according to the present invention, in order to negate the
deleterious effects of iron on the tinplating process without using
ferrocyanide addition to the bath, specific organic compounds and
combinations thereof have been evaluated as additives to the halogen bath
which slow down the depletion of divalent tin (i.e., stannous ions) from
the electroplating process. These chemical additives either stabilize the
ferrous ions in solution so that they are more slowly oxidized to the
ferric ion (i.e., reaction 1 is retarded) or stabilize the stannous ions
so that they react more slowly even in the presence of ferric ions (i.e.,
reactions 3 and 4 are minimized).
The additives found particularly effective in accomplishing the above
include para-aminobenzoic acid, hydroquinone, gallic acid, catechol,
resorcinol, salicylic acid, ascorbic acid (L- or D-), citric acid, oxalic
acid, formic acid, acetic acid, tartaric acid, glycine, diethyl
hydroxylamine (DEHA), mixtures of citric acid and hydroquinone, and
mixtures thereof.
A method for minimizing the effect of dissolved iron in high-speed tin
electroplating comprises incorporating the chemical additives discussed
above in a weakly acidic halogen bath solution in an amount sufficient to
substantially inhibit soluble ferrous ions, ferric ions, and stannous ions
from reacting in the plating bath. As a result, these additives minimize
the formation of stannic tin without the need for ferrocyanide additions.
The following examples illustrate the present invention. Halogen bath tin
oxidation studies were performed in which oxygen was bubbled through 150
mL of a halogen bath plating solution containing 19.6 gms/l NaHF.sub.2,
26.5 gms/l NaF, 12.68 gms/l NaCl, and 33.0 gms/l SnF.sub.2, together with
1 ml/l of a commercial grain refining additive known in the trade as Agent
20, in a 250 mL graduated cylinder immersed in a water bath at 140.degree.
F. The electrolytes used for these experiments were typical of halogen
tinplating baths and contained 25 g/l of total tin to initially yield
20-24 g/l of stannous ions. The mole ratio of fluoride to total tin was
8:1 and the initial pH of the solution was 3.4-3.5. The concentration of
the addition agents in the experimental baths was set at 0.1 molar unless
otherwise specified. Iron (1 g/l) was added at the beginning of the test
as ferrous sulfate crystals (FeSO.sub.4.7H.sub.2 O). To accelerate the
oxidation process, either air or pure oxygen gas was sparged from the
bottom of the container at 120 cc/min. The concentration of stannous ions
was determined by titration every thirty or sixty minutes during a three
or eight hour test period.
Tables 1 and 2 below tabulate the results of the oxidation studies using
the additives of the present invention with Table 2 focused on ascorbic
acid as a preferred additive. The effect of L- and D- ascorbic acid
additives upon the oxidation of stannous ions was evaluated both with and
without the addition of ferrous ions in the bath. Generally, the data show
the depletion of stannous ions by oxidation in both cases. This oxidation
is exacerbated, however, when ferrous ions are added to the bath. In order
to determine the effectiveness of the additives, comparative examples were
also run on baths free of addition agents both with and without ferrous
ions in the bath. Also included were examples which included ferrocyanide
additions. Table 3 tabulates results of oxidation studies on a halogen
bath where either no additives, sodium ferrocyanide, or nitrogen were
introduced into the bath.
In comparing Tables 1 and 2 with Table 3, the loss of stannous ions is seen
to be lower when the additives of the present invention are incorporated
into the bath than when no additions are made at all. In particular, the
use of ascorbic acid as an additive is particularly effective. A
comparison of run 3 of Table 2 with run 20 of Table 3 shows that a low
concentration (e.g., 0.005 M) of L-ascorbic acid results in more than four
times the concentration of stannous ions in the bath after 180 minutes of
oxygen injection in the presence of iron added to the bath at an initial
concentration of 1 g/l. An increase in the L-ascorbic acid concentration
to 0.05 M (run 25 of Table 1) increases the amount of stannous ions to
almost six times after 180 minutes of oxygen injection. Similarly,
D-ascorbic acid, when used at a concentration off 0.05 M (run 27 of Table
1), increases the amount of stannous ions to over 6 times after 180
minutes of oxygen injection.
The additives used in Tables 1 and 3 have been abbreviated as follows:
PDHB=hydroquinone; MDHB=resorcinol;
ODHB=catechol; THBA=gallic acid;
AAA=glycine; DEHA=diethyl hydroxylamine;
PABA=para aminobenzoic acid; EDTA=ethylene diamine tetra acetic acid; and
AA-55 is known in the art as the designation for sodium ferrocyanide.
TABLE 1
__________________________________________________________________________
PRESENT INVENTION
Fe.sup.+2 initial
final
Stannous Ion Concentration (g/l)
Run
Addition Agents
added g/l
Oxidant Temp. F.
pH pH 0.0
30.0
60.0
90.0
120.0
150.0
180.0
(Min.)
__________________________________________________________________________
1 0.1M oxalic acid
0.000
O.sub.2 120 ml/min
145 3.5 -- 19.3
17.4
15.7
13.8
12.1
10.5
8.4
2 0.1M oxalic acid
0.040
O.sub.2 120 ml/min
145 3.5 -- 18.6
13.3
11.4
9.3
7.6
6.2
4.6
3 0.1M citric acid
0.000
O.sub.2 120 ml/min
145 3.5 -- 20.0
18.6
16.8
15.0
13.2
11.7
10.0
4 0.1M citric acid
1.000
O.sub.2 120 ml/min
145 3.5 -- 18.8
14.0
12.4
10.7
9.3
7.9
6.4
5 0.1M PDHB 1.000
O.sub.2 120 ml/min
140 -- 4.0
19.0
15.5
15.0
13.6
12.6
11.7
10.7
6 0.1M PAA 1.000
O.sub.2 120 ml/min
140 -- 4.5
18.2
15.5
12.0
8.1
4.3
2.0
0.0
7 0.1M EDTA 1.000
O.sub.2 120 ml/min
140 3.5 4.0
21.9
13.6
10.7
8.1
6.0
4.1
1.9
8 0.02M PDHB
1.000
O.sub.2 120 ml/min
140 3.5 4.4
21.9
15.7
14.3
12.6
11.2
9.5
7.9
9 0.05M PDHB
0.000
O.sub.2 120 ml/min
140 3.5 4.3
21.7
20.7
20.2
19.3
18.3
17.6
16.9
10 0.05M PDHB
1.000
O.sub.2 120 ml/min
140 3.5 4.5
21.7
17.6
16.7
15.2
14.0
12.6
11.4
11 0.05M PDHB
0.500
O.sub.2 120 ml/min
140 3.4 4.4
24.5
21.4
19.8
19.0
17.6
16.9
16.0
12 0.05M PDHB
0.200
O.sub.2 120 ml/min
140 3.4 4.4
24.5
21.7
20.7
19.8
19.0
18.1
17.1
13 0.05M MDHB
0.000
O.sub.2 120 ml/min
140 3.4 4.5
25.0
23.8
22.9
21.2
19.8
18.3
17.4
14 0.05M MDHB
1.000
O.sub.2 120 ml/min
140 3.4 4.5
25.0
18.3
16.9
15.0
13.6
11.9
11.0
15 0.05M ODHB
0.000
O.sub.2 120 ml/min
140 3.4 4.0
24.5
22.9
22.4
21.2
20.5
19.8
19.0
16 0.05M ODHB
1.000
O.sub.2 120 ml/min
140 3.4 4.3
24.5
18.3
17.4
16.7
16.0
13.1
12.6
17 0.05M 3,4,5 THBA
0.000
O.sub.2 120 ml/min
140 3.4 4.1
24.5
23.6
22.6
21.4
20.7
19.5
18.3
18 0.05M 3,4,5 THBA
1.000
O.sub.2 120 ml/min
140 3.4 4.3
24.5
19.8
18.8
17.1
15.2
14.3
13.1
19 0.1M EDTA 0.000
O.sub.2 120 ml/min
140 3.4 4.1
24.5
22.6
20.9
19.5
17.6
16.4
14.8
20 0.05M AAA 0.000
O.sub.2 120 ml/min
140 3.4 4.4
24.5
22.9
21.2
19.8
17.9
16.2
14.5
21 0.05M AAA 1.000
O.sub.2 120 ml/min
140 3.4 4.7
24.5
15.0
13.8
12.6
11.9
11.2
10.0
22 0.10M PDHB
0.000
O.sub.2 120 ml/min
140 3.4 3.7
24.5
23.6
23.1
22.6
22.4
21.9
21.7
0.10M citric acid
23 0.10M PDHB
1.000
O.sub.2 120 ml/min
140 3.4 3.7
24.5
21.4
20.9
20.5
19.5
19.3
18.8
0.10M citric acid
24 0.05M L-ascorbic
0.000
O.sub.2 120 ml/min
140 3.4 4.2
22.4
23.8
23.6
22.9
21.7
21.5
21.0
25 0.05M L-ascorbic
1.000
O.sub.2 120 ml/min
140 3.4 4.2
22.4
22.4
21.3
20.8
19.8
19.1
18.4
26 0.05M D-ascorbic
0.000
O.sub.2 120 ml/min
140 3.4 3.7
24.2
26.1
25.4
25.2
25.0
24.2
24.0
27 0.05M D-ascorbic
1.000
O.sub.2 120 ml/min
140 3.4 3.7
24.2
23.5
23.4
22.8
22.3
21.0
20.9
28 0.05M PABA
0.000
O.sub.2 120 ml/min
140 3.4 4.1
24.1
22.4
20.7
19.3
17.8
16.1
14.6
29 0.05M PABA
1.000
O.sub.2 120 ml/min
140 3.4 4.4
24.1
19.1
17.5
16.3
15.2
13.9
12.9
30 0.05M salicylic
0.000
O.sub.2 120 ml/min
140 3.4 4.2
24.1
22.1
19.8
17.7
15.1
13.5
12.0
31 0.06M salicylic
1.000
O.sub.2 120 ml/min
140 3.4 4.4
24.1
16.5
14.4
12.5
10.4
9.2
7.6
32 0.05M DEHA
0.000
O.sub.2 120 ml/min
140 3.4 4.5
22.2
20.7
19.7
18.4
17.5
16.9
15.8
33 0.05M DEHA
1.000
O.sub.2 120 ml/min
140 3.4 4.7
22.2
15.8
15.4
14.5
13.7
13.2
12.9
__________________________________________________________________________
TABLE 2
__________________________________________________________________________
Fe.sup.+2
Addition added Temp.
initial
final
Stannous Ion Concentration g/l)
Run
Agents
g/l Oxidant F. pH pH 0 60 120
180
240
300
360
420
480
(Min.)
__________________________________________________________________________
1 0.010M
1.000
O.sub.2 120 ml/min
140 -- 4.6
25.2
19 17.7
16.9
15.4
14.3
13.1
11.9
10.5
L-ascorbic
2 0.025M
1.000
O.sub.2 120 ml/min
140 -- 4.4
25.2
21.4
20 18.8
17.3
16.4
14.7
13.3
11.9
L-ascorbic
3 0.005M
1.000
O.sub.2 120 ml/min
140 -- 5 23.8
17.3
15.8
14.3
12.7
11.4
10 8.8
6.7
L-ascorbic
4 0.05M 1.000
O.sub.2 120 ml/min
140 3.4 4.2
22.4
21.3
19.8
18.4
-- -- -- -- --
L-ascorbic.sup.1
5 0.05M 1.000
O.sub.2 120 ml/min
140 3.4 3.7
24.2
23.4
22.3
20.9
-- -- -- -- --
D-ascorbic.sup.2
6 none 0.000
O.sub.2 120 ml/min
140 3.4 4.7
23.8
20.9
18.1
15.9
13.4
11.3
10.1
9.4
9
7 none 1.000
O.sub.2 120 ml/min
140 3.4 4.6
23.7
12.1
7.1
3.3
-- -- -- -- --
__________________________________________________________________________
.sup.1 Run 25 Table 1
.sup.2 Run 27 Table 1
TABLE 3
__________________________________________________________________________
Fe.sup.+2 initial
final
Stannous Ion Concentration (g/l)
Run
Addition Agents
added g/l
Oxidant Temp. F.
pH pH 0.0
30.0
60.0
90.0
120.0
150.0
180.0
(Min.)
__________________________________________________________________________
1 none 0.000
O.sub.2 120 ml/min
145 4 -- 20.8
19.0
17.6
15.5
13.3
11.4
9.5
2 none 1.000
O.sub.2 120 ml/min
145 4 -- 20.0
11.4
9.0
6.4
3.8
2.4
1.4
3 3 g/l AA-55
0.000
O.sub.2 120 ml/min
140 3.4 4.7
24.5
23.0
21.9
20.7
19.5
17.9
17.1
4 3 g/l AA-55
1.000
O.sub.2 120 ml/min
140 3.4 4.8
24.5
17.1
16.0
14.3
13.1
12.4
11.2
5 none 0.000
O.sub.2 120 ml/min
140 3.4 4.5
24.8
22.6
20.5
18.6
17.1
16.2
13.1
6 none 0.200
O.sub.2 120 ml/min
140 3.4 4.7
24.8
19.3
17.6
16.0
14.0
11.7
10.2
7 none 0.000
AIR 120 ml/min
140 -- 3.7
24.0
24.0
23.8
23.3
23.3
22.9
22.6
8 none 0.056
AIR 120 ml/min
140 -- 3.8
24.0
23.3
22.9
22.6
22.1
21.7
21.2
9 none 0.113
AIR 120 ml/min
140 -- 3.8
24.0
22.9
22.4
21.7
21.4
20.7
20.5
10 none 0.565
AIR 120 ml/min
140 -- 3.9
24.0
20.7
19.8
19.3
18.6
18.6
17.9
11 none 0.000
AIR 120 ml/min
140 -- 4.1
24.3
24.0
24.0
23.8
23.6
23.1
23.1
12 none 1.000
AIR 120 ml/min
140 -- 4.4
24.3
19.5
18.6
17.9
17.1
16.9
16.4
13 none 0.000
AIR 120 ml/min
140 3.4 3.7
23.9
23.6
23.5
23.4
23.0
22.9
22.8
14 none 0.056
AIR 120 ml/min
140 3.4 3.8
23.9
22.0
21.0
21.0
20.7
20.7
20.6
15 none 0.113
AIR 120 ml/min
140 3.4 4.0
23.9
20.6
20.2
20.2
19.6
19.6
19.3
16 none 0.226
AIR 120 ml/min
140 3.4 4.2
23.9
22.0
20.4
19.8
19.8
19.7
19.6
17 N.sub.2 120 ml/min
0.000
none 140 3.4 3.4
23.0
22.9
22.9
22.9
22.9
22.9
22.9
18 N.sub.2 120 ml/min
0.565
none 140 3.4 3.5
22.9
22.2
22.2
22.2
22.2
22.2
22.2
19 N.sub.2 120 ml/min
0.565
none 140 3.4 3.6
22.9
21.6
21.6
21.7
21.8
21.7
21.8
Fe.sup.+3
20 none 1.000
O.sub.2 120 ml/min
140 3.4 4.6
23.7
14.9
12.1
9.2
7.1
4.8
3.3
21 none 0.000
O.sub.2 120 ml/min
140 -- 3.8
25.2
22.0
19.1
16.6
14.7
14.0
13.8
22 none 0.000
O.sub.2 120 ml/min
140 -- 3.7
25.1
23.5
21.9
20.6
19.3
17.7
16.4
__________________________________________________________________________
From the data in Table 1, it has been determined that when the stannous ion
concentration is in the range of 8 to 25 g/l the stannous ion oxidation
rate is independent of the stannous ion concentration. Thus the change in
stannous ion concentration, .DELTA.Sn.sup.+2, of the data is discussed
below and graphically displayed in FIGS. 2-7 in order to eliminate the
need to consider small variations in the starting stannous ion
concentrations. It should be noted that, for all of the additives of the
present invention, the .DELTA.Sn.sup.+2 versus time curves shown in FIGS.
3-7 exhibit basically the same shape, i.e., a sudden decrease in the
stannous ion concentration in the first thirty minutes (represented in the
graphs by a sharp increase in the slope of the .DELTA.Sn.sup.+2 versus
time curves) followed by a linear region of decreasing stannous ion
concentration (represented in the graphs by a continued increase in the
slope of the .DELTA.Sn.sup.+2 versus time curves but at a more gradual
rate) similar to that which results in the absence of iron.
It is believed that this initial deleterious effect of iron in increasing
the oxidation rate of stannous ions corresponds to reaction 3 discussed
above and diminishes quickly as the stable sodium fluoferrate compound is
formed according to reaction 2 above. It is also theorized that the linear
region of decreasing stannous ion concentration corresponds to the
continued direct oxidation of stannous ions in solution according to
reaction 4 above. The data of Tables 1, 2, and 3 were used to prepare the
graphs of FIGS. 2-7 which serve to further explain the present invention
with regard to the effect of the additives of the present invention upon
stannous ion depletion by both of these mechanisms.
FIG. 2 shows, as a comparative baseline, the effect of oxygen on the
stannous tin concentration, without iron added and with iron added in
concentrations of 0.2 g/l (200 ppm) and 1.0 g/l (1,000 ppm) as solid
ferrous sulfate (FeSO.sub.4.7H.sub.2 O) at the beginning of the
experiment. The effect of iron on accelerating the oxidation of Sn.sup.+2
is very strong and occurs during the initial minutes of oxygenation (i.e.,
during the first 30 minutes of oxygenation), after which the oxidation
rate is essentially the same as without iron. Furthermore, in the case of
iron additions, a white powdery precipitate formed during the initial
minutes. This powder was identified as the complex ferric salt, sodium
fluoferrate (Na.sub.3 FeF.sub.6), by X-ray diffraction and by Energy
Dispersive Spectroscopy.
FIG. 3 shows the effect of 0.10 molar concentrations of carboxylic acids,
namely acetic, oxalic, citric, formic, and tartaric acids, on the
oxidation of stannous tin. Citric acid reduced the initial effect of iron
by almost 50% and also reduced the continued direct oxidation of stannous
ions, as evidenced by the change in slope of the linear portion of the
curve. Oxalic acid was nearly as effective in mitigating the iron effect,
but did not have as strong an effect on continued direct stannous
oxidation. The impact of the other carboxylic acids, although not as
great, still shows an improvement over using no addition agent at all.
FIG. 4 shows the effect of 0.05 molar concentrations of six aromatic
compounds, namely, gallic acid, hydroquinone and its two isomers
(resorcinol and catechol), para-aminobenzoic acid, and salicylic acid. All
of these additives reduced the initial iron effect to a certain extent and
most had a strong impact on the continued direct oxidation of stannous
ion. Hydroquinone had a very strong effect in reducing both the initial
impact of iron and the continued direct oxidation of stannous ions. Gallic
acid and para-aminobenzoic acid were also nearly as effective as
hydroquinone. Catechol, resorcinol, and salicylic acid were also found to
have the effect of lowering the oxidation rate of stannous ions. Thus,
aromatic compounds were found to be effective in both reducing the initial
effect of iron and lowering the continued direct oxidation rate of
stannous ions in the plating bath.
FIG. 5 shows the profound effect 0.05 M L-ascorbic acid have on both the
initial effect of iron and the continued direct oxidation of stannous ions
in the plating bath. It can be seen, by the negative slope in the curve in
the case where no iron is added to the bath, that the stannous ion
concentration increases during the first thirty minutes of oxygenation
when using L-ascorbic acid. It is believed that ascorbic acid actually
reacts with up to an equivalent weight of stannic ions which may be
present in the bath, reducing them back to stannous ions. When iron is
added to the bath, the ascorbic acid eliminates the initial deleterious
effect and also has a very strong impact on the continued direct oxidation
of stannous tin. Moreover, as shown in the tables it should be noted that
the addition of 0.05 M L- or D-ascorbic acid makes a bath with 1.0 g/l of
iron more stable than even a bath containing no iron and no additives.
FIG. 6 shows the effect of aliphatic compounds, namely, 0.05 M additions of
glycine (aminoacetic acid), 0.10 M ethylenediaminetetraacetic acid (EDTA),
and 0.05 M N, N diethylhydroxylamine (DEHA). DEHA reduced both the initial
iron effect and the continued direct oxidation of the stannous ion in the
plating bath. Although glycine was found to slightly increase the initial
iron effect, this compound was found to be effective in reducing the
continued direct oxidation of stannous ions. EDTA did not greatly impact
the initial iron effect or the continued direct oxidation of the stannous
ions in the bath.
FIG. 7 shows the combined effects of citric acid, the most effective of the
carboxylic acids tested above, and hydroquinone, the most effective
aromatic compound tested above. The effects of adding a combination of 0.1
citric acid and 0.1 hydroquinone are a very sharp decrease in the initial
effect of iron and a large reduction in the continued direct oxidation of
stannous ions in the bath.
Although the additives (and methods for using them) disclosed above are
effective when used alone or in combination to stabilize the stannous tin
concentrations in halogen plating baths, they may also be used in
conjunction with other iron removal technologies. It is contemplated that
these addition agents, when used in this manner, may stabilize iron ions
in the plating bath for a time sufficient to permit circulation of the
bath through an iron removal apparatus, such as ion exchange columns or
molecular recognition technology, to remove them from a plating bath.
Because these iron removal systems require time to process the bath, it is
believed that the addition agents disclosed will permit the use of these
slower iron removal systems in a high-speed plating line which in the past
required that iron be removed rapidly from the plating bath (i.e., with
sodium ferrocyanide).
It is also within the scope of the present invention to use the additives
to stabilize non-halogen tin plating baths.
While the invention has been described herein with reference to specific
embodiments, it is not limited thereto. Rather it should be recognized
that this invention may be practiced as outlined above within the spirit
and scope of the appended claims, with such variants and modifications as
may be made by those skilled in this art.
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